In a typical sputter coating process, an electric discharge produces electrons that collide with a sputtering gas, thereby ionizing the gas. This sputter process is typically in a pressure range between 10 Pa and 0.1 Pa. In this pressure range the number of atoms or molecules is between 5*1015 cm−3 and 2*1013 cm−3. As ions bombard a target, atoms are detached from the target, the atoms deposit at a substrate to be coated. The process of detaching atoms from the target is called sputter process. This sputter process can also be used for etching. In some systems, improved target utilization and coating uniformity may be achieved by producing a highly ionized plasma in the vicinity of the target. In such systems, a partially ionized plasma is first generated at a low voltage, and then a highly ionized plasma is generated through the application of high power discharge pulses. A highly ionized plasma in this context is reached, when the number of ions is above 1012 cm−3.
However, a high power discharge pulse in combination with an upstream low ionization step has been found to reduce the adhesion of the film produced during the sputtering and may result in target poisoning during reactive sputtering processes. Furthermore, the need to apply a low voltage during the first period of time limits the simultaneous use of the power supply for other purposes, for example, etching.
In EP 1 560 943 B1 a two-step approach for creating a highly ionized plasma is described. For a first period of time a low voltage is applied to a discharge gap and then for a short period of time a higher voltage is applied. This leads at first to a low ionization of the gas and then to a high ionization of the gas.